The present invention relates generally to signal processing systems and more particularly to a method and apparatus for processing a series of digital signals representing a stroke of a writing instrument, such as a stylus, on a writing surface, such as a digitizing tablet, to remove signals at the ends of the stroke caused by retracing.
In many applications a digitizing tablet is used to convert pen or stylus motion into a set of electrical data which is then processed by digital equipment. Typically, there is a special electronic or electromagnetic grid or surface which detects the X and Y positions of the pen as it moves along the surface at a periodic rate. The information is present as two digital data words at a periodic clock rate. This class of technology is used for signature verifications, automatic drafting,graphics, character recognition and so forth. In each case, the user writes on the writing surface with the writing instrument and the position is monitored electronically.
In connecting such a device directly to a processing system there are problems because the raw data may contain certain kinds of noise of other defects which can adversely effect applications that process the data for editing, character recognition, graphics and other uses. The noise may be electrical or mechanical noise produced by the equipment employed to generate the sequence of signals corresponding to the stroke of the writing instrument. The noise may also be natural noise produced by the writer if he retraces a portion of the stroke at either end.
For example, instead of forming the left leg in an upper case letter N by starting at the bottom left and moving the writing instrument upward the writer may start at the top left and make a "down" line first and then an "up" line which is more or less over or very close to the "down" line. As can be appreciated, in a character recognition system this latter method of writing the letter N may, when processed, produce an upper case letter W. Furthermore, in graphics applications portions of strokes caused by retracing will detract from the intended shape of the resulting image.
As can be appreciated, it would be desirable to be able to detect and remove data signals at the ends of a stroke which are caused by retracing.
In U.S. Pat. No. 4,375,081 to B. Blesser there is described a technique for removing or minimizing noise over the length of a stroke caused by writing slowly. The technique comprises filtering a signal represented by a first series of indicia where each indicium of the series represents an amplitude by serially averaging the amplitudes of sets of n indicia of the series to form a second series of indicia and serially amplitude comparing each subsequently occurring indicium thereof to form a third series of indicia which includes those indicia resulting from comparisons having an amplitude difference greater than a predetermined amount.
In U.S. Pat. 4,284,975 to K. Odaka there is disclosed a pattern recognition system for hand-written characters operating on an on-line basis comprising a character input unit for providing the coordinates of a plurality of points on strokes of a hand-written input characters, an approximate unit for providing some feature points for each stroke of the input character, a pattern difference calculator for providing the sum of the length between the feature points of the input character and those of the reference characters which are stored in the reference pattern storage, and a minimum difference detector for determining the mimimum value of the difference among the pattern differences thus calculated and determining the input character as the reference character which provides the minimum difference.
As far as is known, nothing has been done in the past to eliminate noise caused by retracing.
It is an object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to remove noises which may be present at the start and finish of the stroke.
It is another object of this invention to provide a new and improved technique for processing a series of signal representing a stroke of a writing instrument on a surface to remove noise which may be present at the start and finish of the stroke caused by retracing.
It is still another object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to determine whether any of the signals at the ends are as a result of retracing.
It is yet still another object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to remove noise which may be present at the start and finish of the stroke caused by retracing which is not dependent on the rate at which the signals representing points aong the stroke are sampled and does not require that the points be uniformly spaced from one another.
It is a further object of the invention to provide a technique as described above which does not alter the signals between the points determined to be the end points.